Automated clinical chemistry analyzers are routinely used to assay the concentrations of analytes in patient samples such as blood, urine, and spinal column fluid. Typically, the patient samples are mixed with reagents and the resulting reactions are monitored using one of several well-known techniques, including colorimetry, ion selective electrodes or nephelometry, and rate methods using such analytical techniques.
It is well known in the field of clinical chemistry that a reaction may be influenced by the temperature at which the reaction is performed. If the temperature of the reaction varies, the rate of the reaction or the quantity of reaction product may also vary. The results could thus be inconsistent with previous assays or with the results of calibration reactions used to establish a calibration relationship for the assay.
The components that comprise a typical clinical chemistry reaction include one or more reagents and a patient sample. Often the reagents are refrigerated at approximately 2.degree. to 15.degree. C. while the samples are generally at room or ambient temperature of about 17.degree. to 27.degree. C. It is common, however, to perform clinical chemistry reactions at either 30.degree. C. or 37.degree. C. Thus, it is necessary to raise the temperature of both reagents and sample to the predetermined reaction temperature and then hold the temperature constant throughout the reaction. Because instrument throughput depends upon the number of samples that may be processed within a given time period, it is most advantageous to adjust the reagent and patient sample temperatures as quickly as possible to the reaction temperature.
There are various techniques and devices used for adjusting the temperature of reagents and samples and thereafter controlling the reaction temperature on clinical chemistry instruments. For example, it is known to use individual reaction heating coils around individual reaction vessels or cuvettes. With such individual reaction vessels, it is also known to preheat the reagent delivered into the reaction vessels so that the time required for the reagent to reach the predetermined reaction temperature is decreased. See, for example, U.S. Pat. No. 4,086,061, entitled "Temperature Control Systems for Chemical Reaction Cell" filed in the name of Hoffa et al.
Although such an approach is feasible for a relatively few number of individual reaction vessels, such an approach becomes cumbersome when the contents of a large number of reaction vessels or cuvettes are to be simultaneously assayed. To overcome this disadvantage, it is known to use circulating heated air or water baths which flow about the reaction vessels. Using such a technique, the temperature of a large number of reaction vessels or cuvettes can be simultaneously controlled.
While a circulating air or water bath can control the temperature of a large number of reaction vessels simultaneously, the rate at which heat transfers from such a bath to a reaction vessel and its contents is substantially proportional to the difference between the temperature of the vessel and the temperature of the bath, to the heat capacity of the fluid, and to the efficiency of the contact therebetween.
For example, the time required for a "perfect" heat source to change the temperature of a reaction cuvette from 27.degree. C. to 36.degree. C. is the same as the time required to change the reaction cuvette from 36.degree. C. to 36.9.degree. C. and to change from 36.9.degree. C. to 36.99.degree. C. With other than "perfect" heat sources, that is, essentially all practical systems, the time required for temperature changes is even longer because the heat source temperature varies with the thermal loading presented by the contents of the reaction cuvette.
In addition to the fundamental thermodynamic difficulties just discussed in using circulating fluid baths, air and water, the two common fluids used, both present further drawbacks and disadvantages. More particularly, the specific heat of air is so small that it becomes very difficult to control the temperature of reaction cuvettes to within a small part of a tenth of a degree Celsius. Thus, air is essentially useless as a thermal control fluid in clinical analyzers.
While water has a superior specific heat as compared to air, water tends to readily support the growth of algae, requiring the use of growth inhibiting agents and regular and generally burdensome routine maintenance. Furthermore, water must be rapidly moved about the reaction cuvettes to provide a suitably efficient contact between the water and the cuvettes if narrow temperature tolerances are to be maintained.
In addition to fluid baths, it is also commonly known to install reaction cuvettes in thermal contact with a temperature controlled body or mass having good thermal conductivity and a specific heat as high as practical. For example, a plurality of reaction cuvettes may be located in cavities within an aluminum or copper body. The temperature of such a body is controlled to within less than few hundredths of a degree Celsius under steady state conditions, that is, when no fluids or cuvettes are being added to or withdrawn from the body. However, when fluids other than the temperature of the body are added to cuvettes already installed on the body, or when fluid filled cuvettes are installed, a localized temperature change results. The heater controller which controls a heating element used to maintain the body at the predetermined temperature responds by altering the power input to the heating elements to restore the average temperature of the body. Unfortunately, such a system may result in temperature over-shoot in other regions of the body because the temperature controller senses and controls only the average body temperature.
Thus, the various temperature control techniques known in the art each have inherent drawbacks and disadvantages relating to the time required for the contents of a reaction cuvette to come to the desired analysis temperature. Unfortunately, the time required for the temperature difference to be narrowed to the required reaction temperature directly impacts and influences the automated analyzer throughput. Where rapid sample analysis and high throughput are desired, the time required for the reaction cuvettes to be brought to the reaction temperature can be a large percentage of the time allowed for the various chemical analyses to be performed.
Thus, there is a need for an improved apparatus for adjusting and controlling the temperature of reaction cuvettes within automated clinical analyzers.